1. Field of the Invention
The present invention relates generally to sterilization mixtures, and more particularly to sterilization mixtures comprised of ethylene oxide and 1,1,1,2,3,3,3-heptafluoropropane.
2. Description of the Prior Art
Sterilization technology is employed in a wide range of industries and because of this widespread use a variety of sterilization techniques have been developed. One common method of sterilization is dry-heat sterilization, in which the items to be sterilized are heated to a temperature of about 160.degree. C. to 170.degree. C. for several hours. Specific exposures are dictated by the bioburden presented, with higher temperatures requiring less exposure time.
A second common sterilization method is steam sterilization in which a moist heat is applied. Steam sterilization at 100.degree. C. and atmospheric pressure is generally not effective, so elevated temperatures and/or pressures are used. Appropriate temperatures range from about 115.degree. C. to 125.degree. C. or more; pressures range from atmospheric to about 100 psi. The process is most commonly carried out in autoclaves using both elevated pressures and saturated steam.
Although the dry-heat and steam sterilization methods are broadly applicable, certain articles, and particularly those employed in the medical and aerospace industries, cannot withstand the high temperatures of dry-heat sterilization or the moisture of steam sterilization. To sterilize these articles a variety of gaseous sterilants have been developed. These gaseous sterilants typically function at relatively low temperatures and normally comprise anhydrous compositions.
Ethylene oxide is widely employed as the sterilizing agent in sterilization compositions due to its overall effectiveness in that regard. The use of ethylene oxide however is known to have both advantages and disadvantages. One advantage of ethylene oxide is the volatility of its residues. Because ethylene oxide residues volatilize quickly they are less likely to be absorbed by or adsorbed to the articles being sterilized.
One disadvantage of using ethylene oxide in sterilant compositions is its flammability. This flammability requires ethylene oxide to be employed in a carefully controlled manner or to be combined with a fire retardant material. In practice, ethylene oxide is generally not used alone for sterilization. Most often, ethylene oxide is employed in admixture with a fire suppression agent such as carbon dioxide or a fluorocarbon gas.
Over the past several decades, the most commonly employed fire suppression agent for ethylene oxide-based sterilizations mixtures has been the chlorofluorocarbon dichlorodifluoromethane (CFC-12). For example, one commonly employed sterilant mixture comprises 27.3 mole percent (12 weight percent) ethylene oxide and 72.7 mole percent (88 weight percent) CFC-12 and is commonly referred to as 12-88 in the industry. Similar CFC-containing sterilants are also known.
Recent studies have suggested that chlorine-containing compounds such as chlorofluorocarbons and hydrochlorofluorocarbons release chlorine atoms to the stratosphere and may be at least partially responsible for the observed deterioration of the earth's protective ozone layer. Accordingly, the production and use of chlorofluorocarbons, including CFC-12, is being severely restricted and may eventually be banned entirely. Thus, fire suppression agents for ethylene oxide-based sterilizations mixtures that do not contain chlorofluorocarbons are desired for their compatibility with current environmental concerns.
Carbon dioxide has been proposed as one non-CFC-containing diluent for ethylene oxide. Because of its poor inerting characteristics however, a nonflammable mixture of carbon dioxide and ethylene oxide contains less than 40 percent of ethylene oxide per unit volume. As pointed out by Chippett et al. in U.S. Pat. No. 5,039,484, sterilization must therefore be carried out either at higher pressures or for longer contact times.
A further problem associated with the use of carbon dioxide arises from the large difference in boiling points between it and ethylene oxide. This leads to fractionation of the mixture upon withdrawal from the storage tank, and increases the danger of delivering a sterilant mixture rich in either carbon dioxide or ethylene oxide. If the mixture is rich in carbon dioxide it will have poor sterilization characteristics. If it is rich in ethylene oxide it may be explosive.
A need therefore exists for ethylene oxide-based sterilization agents that are safe and effective to use and are environmentally acceptable. The present invention addresses that need.